Wordpress, Joomla, & Drupal are the big ones. You can get extensions to do almost anything. And if you can't you can extend them yourself. Would save you weeks of work!
HTML based isn't really the implementation you should be using for such. This would require being IE only due to file system restrictions. Also, being client-side, anyone could easily alter code.
Simple question: Why reinvent the wheel? There are 3 really huge/popular/powerful/extendable CMSs out there. And if you bring in the smaller names, then it adds up quick.
All a form does is post data to a server-side authentication system. AJAX can do the same thing. It's still posting data just as a form does. Just figure out where it's going and you can post to it....
For future game development...you better pick C#. Remember that ASP.NET is not a language, but an implementation. It's written in VB.NET or C#. So you could eventually kill 2 birds with one stone.
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What programming experience do you have? You are going to need some heavy JavaScripting (if using IE) or Java (if using JavaWS) in order to get this project done.
If you have absolutely no web development experience, why do you actually plan to take on a project you cannot complete? Even a beginner could not handle what all you've laid out. Think about how...
I didn't mention because you called yourself a "very basic coder". Rewriting someone elses extensions would be a pain even for a very seasoned professional. It would first take time to see how the...
Being that it is a CMS based site, the login form, social media icons, and search form are probably all separate extensions. You would probably have to edit the actual extensions to accomplish that.
You could create any type of script to parse out the <script> tags through regular expressions. Install PHP on the local machine and run scripts through the command line.
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If the form processing is done correctly, it will check the HTTP referrer and make sure the form is coming from it's own site, not anyone elses. If that is the case, you will not be able to...
FYI...PERL questions do go here because it is a typical server-side language. Forums aren't divided by implementation, rather the language they are written in.
As the code you posted itself stands, there is no error. The only way we could help better is to have you save an archive of the webpage and upload it here.